1. Field of the Invention
The present invention relates to a polycarbodiimide. More specifically, the present invention relates to a polycarbodiimide and its uses, and a process for preparing the polycarbodiimide.
2. Discussion of the Related Art
An aromatic polycarbodiimide has been used in flameproof films and heat-resistant adhesives since the aromatic polycarbodiimide has excellent heat resistance. As the aromatic polycarbodiimide, those prepared by polymerizing monomers such as diphenylmethane diisocyanate (MDI) and tolylene diisocyanate (TDI) have been known.
A film made of the aromatic polycarbodiimide generally has a characteristic such that a volatile gas or a decomposed monomer is not generated from the film even at high temperatures of not less than 400xc2x0 C.
However, the flexibility of the film is lowered due to self-cross-linking when heat of not less than 200xc2x0 C. is applied to the film for a long period of time, and the film is not suitable for applications requiring bending because its flexural modulus is so high.
In the process for producing a semiconductor device, a semiconductor chip is bonded to a lead frame or an electrode by coating a paste such as silver paste on a die pad of a lead frame, mounting a semiconductor chip thereon, and curing the paste.
However, since the paste cures even at room temperature, the amount of paste coated changes due to the increase of its viscosity and variation in shape of the paste would be caused during coating, so that variation in reliability of the bonding strength of the semiconductor chip would be caused. For instance, when the amount of the paste coated is changed and insufficient, the bonding strength between the semiconductor chip and the electrode is lowered, so that the semiconductor chip can be peeled off during the subsequent wire bonding step. On the other hand, when the amount of the paste: coated is too large, the paste flows onto the semiconductor chip, so that the yield and the reliability can be lowered. Such defects are even more remarkable for large-sized semiconductor chip, and in order to ensure the reliability of the bonding strength, the amount of coating of the paste should be frequently adjusted, so that there often causes hindrance in workability and productivity.
Recently, in order to achieve the improvement in properties of a semiconductor device, there has been employed a process comprising pouring a liquid thermosetting resin so-called xe2x80x9cunderfilling materialxe2x80x9d in a gap between a semiconductor element and a wiring circuit substrate, curing the thermosetting resin to form a resin-cured product, to distribute stress concentrated to an electric connection part into the resin-cured product, thereby improving the reliability of the connection.
However, in order to pour the liquid thermosetting resin between the semiconductor element and the wiring circuit substrate, it is necessary to provide a space for positioning a pouring nozzle near the mounting position of the semiconductor element. With the advancement of miniaturization and thinning of the semiconductor element, it would be difficult to secure the space for positioning a pouring nozzle. Therefore, improvement in the pouring (underfilling) step of the liquid resin has been earnestly desired.
An object of the present invention is to provide a polycarbodiimide which is excellent in flexibility when formed into a film, and maintains low flexural modulus without lowering its flexibility even when the polycarbodiimide is heated at a temperature of not less than 200xc2x0 C. for a long period of time.
Another object of the present invention is to provide an adhesive film for die bonding which gives a stable bonding strength with no variance in the amount of the film during bonding of a semiconductor chip to an electrode and which is excellent in workability and productivity during processing.
In addition, an object of the present invention is to provide an adhesive film for underfilling which is excellent in relaxation of stress caused in the semiconductor element, the wiring circuit substrate and the connecting electrode, and excellent in reliability of electric connection between the semiconductor element and the wiring circuit substrate, and a semiconductor device in which the adhesive film for underfilling is used as a sealing resin.
These and other objects of the present invention will be apparent from the following description.
According to the present invention, there are provided:
(1) a polycarbodiimide represented by the formula (I): 
xe2x80x83wherein R1 is an alkylene group having 2 to 10 carbon atoms, R2 is a divalent aromatic group, R3 is a monovalent aromatic group, k is 0 or an integer of 1 to 30, m is an integer of 2 to 100, and n is 0 or an integer of 1 to 30;
(2) a film made of the above-mentioned polycarbodiimide;
(3) an adhesive film for die bonding, made of the above-mentioned polycarbodiimide;
(4) a semiconductor device comprising a semiconductor chip and an electrode, wherein the semiconductor chip is bonded to the electrode with the aid of the above-mentioned adhesive film for die bonding;
(5) an adhesive film for underfilling, made of the above-mentioned polycarbodiimide;
(6) a semiconductor device comprising a semiconductor element and a wiring circuit substrate, wherein a gap formed between the semiconductor element and the wiring circuit substrate is sealed with the above-mentioned adhesive film for die bonding; and
(7) a process for preparing a polycarbodiimide represented by the formula (I), comprising the steps of:
reacting a polycarbonate diol represented by the formula (II): 
xe2x80x83wherein R1 and m are as defined above, with an aromatic diisocyanate in the ratio of not less than 2 mol of the aromatic diisocyanate per one mol of the polycarbonate diol to give a polyurethane having isocyanate groups at both ends; and
carbodiimidating isocyanate groups existing at both ends of the resulting polyurethane with the residual aromatic diisocyanate in the presence of a catalyst.